The association of AMPK with ULK1 regulates autophagy

Abstract

Autophagy is a highly orchestrated intracellular bulk degradation process that is activated by various environmental stresses. The serine/threonine kinase ULK1, like its yeast homologue Atg1, is a key initiator of autophagy that is negatively regulated by the mTOR kinase. However, the molecular mechanism that controls the inhibitory effect of mTOR on ULK1-mediated autophagy is not fully understood. Here we identified AMPK, a central energy sensor, as a new ULK1-binding partner. We found that AMPK binds to the PS domain of ULK1 and this interaction is required for ULK1-mediated autophagy. Interestingly, activation of AMPK by AICAR induces 14-3-3 binding to the AMPK-ULK1-mTORC1 complex, which coincides with raptor Ser792 phosphorylation and mTOR inactivation. Consistently, AICAR induces autophagy in TSC2-deficient cells expressing wild-type raptor but not the mutant raptor that lacks the AMPK phosphorylation sites (Ser722 and Ser792). Taken together, these results suggest that AMPK association with ULK1 plays an important role in autophagy induction, at least in part, by phosphorylation of raptor to lift the inhibitory effect of mTOR on the ULK1 autophagic complex.

Figure 1. AMPK interacts with ULK1 directly.

(A) 293T cells were transfected with Flag-ULK1 or control empty vector and subjected to immunoprecipitation with anti-Flag antibody, followed by immunoblot analysis with the indicated antibodies. (B) 293T cells were co-transfected with Flag-AMPKα2 and HA-ULK1. Cell lysates were subjected to immunoprecipitation using anti-Flag antibody followed by SDS-PAGE/immunoblot analysis with anti-HA and anti-Flag antibodies. (C) 293T cells were transiently transfected with Flag-AMPKα2 or Flag-AMPKβ1. After 24 h, immunoprecipitation was performed using anti-Flag or control anti-HA antibodies and analyzed by immunoblotting with anti-ULK1 and anti-Flag polyclonal antibodies. (D) 293T cell lysates were subjected to immunoprecipitation with anti-ULK1 polyclonal antibody or control preimmune rabbit serum (NRS), followed by immunoblot analysis with anti-ULK1 and anti-AMPKα antibodies. (E) 293T cells were infected with ULK1 shRNA or control shRNA lentiviruses and subjected to immunoprecipitation with control rabbit IgG or anti-AMPKα antibody followed by immunoblotting with anti-ULK1 and anti-AMPKα antibodies. (F) Purified His6-AMPKα1/β1/γ1 fusion proteins (250 ng) were mixed with purified His6-tagged ULK1 (1 µg) or Bcl-XL (200 ng) proteins in 1% Triton X-100 lysis buffer containing protease inhibitors and subjected to immunoprecipitation with anti-ULK1 antibody. The resulting protein complexes and 10% of the input proteins were analyzed by immunoblotting with anti-His-Tag polyclonal antibody.

Figure 2. AMPK and mTORC1 interact with ULK1 through the PS domain and the kinase domain of ULK1, respectively.

(A) A schematic of ULK1 illustrating its kinase domain, proline/serine-rich domain, C-terminal (CT) domain, and regions for binding to AMPK and mTORC1. (B, C) 293T cells were transfected with empty vector or HA-tagged wild type (WT) or deletion mutants of ULK1 and subjected to immunoprecipitation with anti-HA antibody. The resulting immune complexes were analyzed by immunoblotting with antibodies specific for mTOR, raptor, AMPKα or HA-tag.

Figure 3. The AMPK-ULK1 interaction is important for ULK1-mediated autophagy.

(A) 293T cells were transfected with empty vector or HA-tagged wild type (WT) or deletion mutant (Δ654–828) ULK1 and subjected to immunoprecipitation with anti-HA antibody followed by immunoblotting with the indicated antibodies. (B) U-2OS cells stably expressing GFP-LC3 were infected with lentivirus expressing human Ulk1 shRNA (shhUlk1) or scrambled shRNA (shScr) and subjected to selection with 1 µg/ml puromycin for 5 days. The cells were then transfected with wild type mouse Ulk1 (mUlk1), mUlk1 Δ654–828 or control empty vector for 24 h, incubated in serum-free DMEM overnight, and cultured in complete medium with or without 10 mM metformin for 20 h. The images were obtained using a fluorescence microscope. The scale bars represent 10 µm. (C) The number of GFP-LC3 dots per GFP-positive cell in (B) was quantified (mean ± s.d.; n = 81). (D) U-2OS cells stably expressing GFP-LC3 were transfected with 0, 2 or 4 µg of pcDNA3 encoding HA-tagged ULK1 deletion mutant (554–828). The total amount of plasmid DNA used for each transfection was normalized at 4 µg with empty vector. After 24 h transfection, the cells were starved in nutrient-free medium for 1.5 h, fixed in 4% paraformaldehyde, and analyzed by fluorescence microscopy. The number of GFP-LC3 dots per GFP-positive cell was quantified (mean ± s.d.; n = 100). Statistical significance was determined by Student's t-test and the asterisks indicate P<0.0001.

Figure 4. AICAR induces AMPK phosphorylation of raptor and the recruitment of 14-3-3 to the ULK1-mTORC1 complex.

293 cells stably expressing HA-ULK1 were treated with 1 mM AICAR for the indicated times and subjected to immunoprecipitation with anti-HA or control anti-FLAG antibodies. The resulting immune complexes were analyzed by immunoblotting with the indicated antibodies.

Figure 5. Raptor phosphorylation by AMPK is required for AICAR-induced autophagy in TSC2^−/− MEF cells.

(A) TSC2^−/−, p53^−/− MEFs stably reconstituted with wild-type raptor or S722A/S792A mutant raptor were transfected with GFP-LC3, treated with 2 mM AICAR or control vehicle for 4 h, and then analyzed by fluorescent microscopy. The number of GFP-LC3 dots per GFP-positive cell was counted (mean ± s.d.; n = 60). The scale bars represent 10 µm. (B) TSC2^−/−, p53^−/− MEFs stably reconstituted with wild-type raptor or S722A/S792A mutant raptor were treated with 2 mM AICAR for the indicated times. Cell lysates were prepared in RIPA buffer and analyzed by immunoblotting with the indicated antibodies. The levels of p62 are listed relative to those of untreated WT raptor cells, which were set as 1. (C) Schematic representation of AMPK-mediated suppression of the inhibitory effect of mTORC1 on the ULK1 autophagic complex.